Use of alumina-carbon agglomerates in the carbothermic production of aluminum

ABSTRACT

An agglomerate comprising alumina, carbon, and a binder for use in a vapor recovery reactor of a carbothermic alumina reduction furnace is disclosed. A method for using alumina-carbon agglomerates to capture aluminum vapor species and utilize waste heat from off-gases in a vapor recovery reactor to form a recyclable material is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 11/775,112, filed Jul. 9, 2007, entitled “USE OFALUMINA-CARBON AGGLOMERATES IN THE CARBOTHERMIC PRODUCTION OF ALUMINUM”,which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to methods of and systems and apparatusfor producing aluminum via a carbothermic reactor. In particular, thepresent invention relates to the use of alumina-carbon agglomerates in avapor recovery reactor associated with a carbothermic furnace in theproduction of aluminum.

BACKGROUND OF THE INVENTION

The U.S. aluminum industry is one of the largest in the world with about2.5 million metric tons of primary aluminum produced in 2005. Presently,the aluminum industry relies on three major processes for primaryaluminum production: alumina refining from bauxite, anode production,and aluminum smelting by electrolysis in the Hall process. Hallelectrolytic cells electrochemically reduce alumina to aluminum metalvia carbon anodes and molten aluminum cathodes in the smelting process.Smelting is the most energy intensive step in primary aluminumproduction and accounts for between 2% and 3% of the electricity used inthe U.S. every year (about 15 kWh/kg aluminum produced). Smelting alsoresults in a variety of emissions, effluents, by-products and solidwastes. Greenhouse gases are a major pollutant from aluminum productionand are caused by fossil fuel consumption, carbon anode consumption, andperfluorocarbons from anode effects. Emissions from anode productioninclude particulates, fluorides, polycyclic aromatic hydrocarbons (PAH)and sulfur dioxide (SO₂). Emissions from aluminum smelting includecarbon monoxide (CO), carbon dioxide (CO₂), SO₂, fluorides,perfluorocarbons (PFCs, e.g., CF₄, C₂F₆), and PAH. It would beadvantageous to lower costs and reduce waste to remain competitive withforeign producers. The smelting step is a priority area for improvementbecause of high energy use and undesirable emissions and by-productsimplicated in climate change.

Carbothermic reduction of aluminum is an alternative process foraluminum production. Carbothermic aluminum production involves usingcarbon and temperature changes to effect production of aluminum.Carbothermic processes require much less physical space than the Hallelectrolytic reduction process and could result in decreased electricalconsumption. Long term estimates suggest the carbothermic process couldreduce energy requirement by over 30% to about 8.5 kWh/kg. Carbothermicproduction of aluminum would also eliminate perfluorocarbon emissionsresulting from carbon anode effects, hazardous spent potliners, andhydrocarbon emissions associated with baking of consumable carbonanodes. Thus, carbothermic production of aluminum would be more energyefficient and have less environmental impact than traditional aluminumproduction processes.

The direct carbothermic reduction of alumina to aluminum has beendescribed in U.S. Pat. No. 2,974,032 (Grunert et al.), U.S. Pat. No.4,099,959 (Dewing et al.), U.S. Pat. Nos. 4,033,757; 4,334,917;4,388,107; and 4,533,386 (all Kibby), U.S. Pat. No. 6,440,193 (Johansenand Aune), U.S. Patent Publication No. US2006/0042413 (Fruehan), theProceedings 6^(th) Conference on Molten Slags, Fluxes and Salts, Editedby S. Seetharaman and D. Sichen “Carbothermic Aluminum”, K. Johansen, J.Aune, M. Bruno and A. Schei, Stockholm, Sweden-Helsinki Finland, Jun.12-17, 2002, and “Aluminum Carbothermic Technology Alcoa-Elkem AdvancedReactor Process”, Light Metals 2003, 401-406.

The overall aluminum carbothermic reduction reaction:

Al₂O₃+3C→2Al+3CO   (1)

takes place, or can be made to take place, generally in steps such as:

2Al₂O₃+9C→Al₄C₃+6CO(vapor)   (2)

Al₄C₃+Al₂O₃→6Al+3CO(vapor)   (3)

Al₂O₃+2C→Al₂O(vapor)+2CO(vapor)   (4)

Al₂O₃+4Al→3Al₂O(vapor)   (5), and

Al→Al(vapor)   (6).

A large quantity of aluminum vapor species may be formed during variousones of the above reactions. To recover such vapor species, and thelatent and sensible heat they contain, an external vapor recovery unitor vapor recovery reactor (VRR) may be employed. In the VRR, gasescontaining Al₂O and Al vapors react with carbon to produce Al₄C₃ orAl₄C₃Al₂O slag. Examples of reactions that may occur in the VRR areprovided below:

2Al₂O(g)+5C→Al₄C₃+2CO   (7) 5C≠C₃+C

4Al(g)+3C→Al₄C₃   (8)

Prior methods of recovering Al vapor and Al₂O from off-gases generatedduring carbothermic reduction of alumina are disclosed in U.S. Pat. No.6,530,970 (Lindstad), U.S. Pat. No. 6,849,101 (Fruehan), and Fruehan etal., “Mechanism and Rate of Reaction of Al₂O, Al, and CO Vapors withCarbon”, Metallurgical and Materials Transactions B., 35B, 617-623(2004). Such references generally propose the use of hydrocarbons orcharcoal for reaction with the off-gases. Furthermore, liquidhydrocarbon product may cause bridging of the particles in the reactormaking it difficult to operate the vapor recovery reactor. Solid carbonparticles may also become covered by reaction products, thereby reducingthe reaction rate, eventually resulting in unreacted carbon entering themain carbothermic furnace, which is undesirable. Charcoal has goodsurface area and conversion rates, but is generally four times asexpensive as petroleum products.

SUMMARY OF THE INVENTION

In view of the foregoing, a broad objective of the present invention isto facilitate a more efficient carbothermic aluminum production process.

A related objective is to efficiently capture aluminum vapor andaluminum suboxide vapor off-gases from the carbothermic aluminareduction furnace.

A further related objective is to recover the energy value of theoff-gases.

A further related objective is to increase the efficiency of aluminumcarbide and aluminum carbide slag formation in a vapor recovery reactor.

A related objective is to decrease the heat of the escaping off-gas fromthe vapor recovery reactor.

Yet a further related objective is to reduce the energy required in themain carbothermic furnace.

In addressing one or more of these objectives, the present inventorshave recognized that solely utilizing a carbon feed stream to capturealuminum vapors does not facilitate capture of an economical amount ofaluminum vapor species in the exiting off-gases. To capture more of thealuminum vapor species, it has been recognized that a heat sink may beutilized within the vapor recovery reactor to condense aluminum vaporspecies. It has been further recognized that the heat of condensationfrom the condensation of vapor species may be utilized to drive otherchemical reactions to form materials to be recycled to the carbothermicreactor. Thus, at least some of the energy of the off gas may reclaimed,thereby facilitating more efficient operation of the carbothermicreactor and the corresponding vapor recovery reactor.

In this regard, it has been recognized that alumina-carbon agglomerates(e.g., a pellets) may be utilized as a heat sink. The alumina-carbonagglomerates may be fed to the vapor recovery reactor, wherein hotaluminum gases will condense on the surface of the colder alumina-carbonagglomerates. In turn, the heat of condensation may be utilized to drivechemical reactions between the materials of the agglomerate and/or thealuminum vapor species that result in the formation of recyclablematerial (e.g., slag, aluminum carbide). In one approach, a mixed feedmaterial comprising alumina-carbon agglomerates and additional carbon,for example, carbon rings and/or charcoal briquettes, may be used so asto facilitate capture of aluminum vapor species and production ofrecyclable material. The recyclable material may be added to the mainfurnace to at least partially assist in the production of aluminum. Forexample, the recyclable material may be used during a slag making step.If a relatively high aluminum carbide content is achieved, therecyclable material may be used in the metal making step. Suchmaterials, methods and systems facilitate efficient capture of off-gasesfrom the main furnace and enable capture of at least some of the energyvalue of the off-gas.

In one aspect of the invention, an agglomerate for feeding into a vaporrecovery reactor of an aluminum carbothermic production system isprovided. The agglomerate generally comprises an alumina source, acarbon source, and a non-alkali/non-alkaline binder. The non-alkalinebinder may be an organic binder or an alumina-based binder, such asactivated alumina.

In one embodiment, the agglomerate has a molar ratio of carbon source toalumina source of at least about 3. In one embodiment, the agglomeratehas a molar ratio of carbon source to alumina source of not greater thanabout 4.5. Thus, the agglomerate may have a molar ratio of carbon sourceto alumina source in the range of from about 3 to about 4.5. In relatedembodiments, the weight ratio of alumina source to carbon source is fromabout 2 to about 2.6. In a particular embodiment, the weight ratio ofalumina source to carbon source is about 2.3.

The alumina and carbon sources may be mixed as appropriate to facilitateproduction of the agglomerate. In one approach, the agglomeratecomprises a uniform mixture of the alumina source and the carbon source.In another approach, the agglomerate comprises a core and a shell atleast partially surrounding the core, where the core comprises at leasta portion of the alumina source and the shell comprises at least aportion of the carbon source. In one embodiment, the core consistsessentially of the alumina source and a portion of the binder, and theshell consists essentially of the carbon source and a portion of thebinder. In another embodiment, the core comprises the alumina source, aportion of the carbon source and a portion of the binder, and the shellconsists essentially of at least a portion of the carbon source and aportion of the binder.

For agglomerates comprising an organic binder, the organic bindergenerally comprises less than about 10 weight % of the pellet. In oneembodiment, the organic binder comprises less than about 5 weight % ofthe agglomerate. In one approach, the organic binder comprises apetroleum-based binder (e.g., a carbohydrate-based binder, alignosulfonate salt-based binder). In one embodiment, the organic bindercomprises at least one of coal tar pitch, asphalt, and petroleum pitch.In a particular embodiment, the organic binder consists essentially ofcoal tar pitch, asphalt, and petroleum pitch and combinations thereof.In another embodiment, a carbohydrate-based binders is used, which maycomprise one or more of an aqueous sugar solution, wheat flour, cornflour, corn starch, potato flour, black cane syrup, dextran and dextrinbinders. In a particular embodiment, the organic binder consistsessentially of an aqueous sugar solution, wheat flour, corn flour, cornstarch, potato flour, black cane syrup, dextran, dextrin andcombinations thereof. In another embodiment, the lignosulfonate salt isused as the binder, such as one or more of calcium lignosulfonate,ammonia lignosulfonate, and sodium lignosulfonate. In a particularembodiment, the lignosulfonate salt consists essentially of calciumlignosulfonate, ammonia lignosulfonate, sodium lignosulfonate andcombinations thereof.

The agglomerates may be any suitable shape. In one embodiment, theagglomerates are substantially spherical pellets. In another embodiment,a core of the agglomerate is shaped like a hollow cylinder and is atleast partially surrounded by a carbon shell.

Methods for recovering off-gases from a carbothermic aluminum furnaceare also provided. In one aspect, a method includes the steps of passinga feedstock comprising an alumina-carbon agglomerate through a vaporrecovery reactor, flowing off-gases from a carbothermic reactor into thevapor recovery reactor, treating, concomitant to the passing step, theoff-gases with the feedstock, and recovering at least some recyclablematerial for re-use in the carbothermic aluminum furnace. In aparticular embodiment, the recyclable material comprises at least one ofaluminum carbide and aluminum-carbide containing slag.

In one approach, the treating step comprises heating the feedstock withthe off-gases. In a related approach, the treating step comprisescondensing at least some of the vapors of the off-gas on a surface ofthe alumina-carbon agglomerate. In one approach, the treating stepcomprises reacting aluminum vapor species with carbon-containingmaterials of the alumina-carbon agglomerate, thereby forming at leastsome of the recyclable material. In a related approach, alumina of thealumina-carbon agglomerate is reacted with carbon to form at least someof the recyclable material. In a particular embodiment, thealumina-carbon agglomerate comprises a core and a shell substantiallysurrounding the core, wherein the core comprises alumina and the shellcomprises carbon. In this embodiment, the treating step may include thesteps of reacting the off-gas with the carbon of the shell, exposing thecore of the alumina-carbon agglomerate, and reacting the alumina of thecore with carbon (e.g., the carbon shell or another carbon source).

The alumina and carbon of the alumina-carbon agglomerate may be any ofalumina and carbon sources mentioned herein. The step of passing thefeedstock through the vapor recovery reactor may include the steps offeeding the feedstock from a feed source (e.g., a hopper). In turn, thefeedstock may be pushed toward an inlet of and into the vapor recoveryreactor via a suitable apparatus (e.g., a screw-drive).

In one approach, the method may include the step of feeding, concomitantto the passing step, a separate carbon source into the vapor recoveryreactor. In this approach, the separate carbon source may be utilized topromote condensation of aluminum vapor species and/or promote productionof recyclable material via reaction of the separate carbon source withthe aluminum vapor species and/or the alumina-carbon agglomerates. Theseparate carbon feed may comprise a porous carbonaceous material,thereby facilitating mass transfer within the vapor recovery reactor. Inone embodiment, the separate carbon feed comprises one or more of thecarbon sources utilized to make the alumina-carbon agglomerate. In aparticular embodiment, the separate carbon feed comprises at least oneof a sphere, a briquette, a ring, and a cylinder. In another embodiment,the separate carbon source is generated in situ in the vapor recoveryreactor (e.g., production of charcoal from wood).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described with reference to the accompanyingnon-limiting drawings in which:

FIG. 1 illustrates a schematic view of one embodiment of a carbothermicaluminum production system.

FIG. 2 is a schematic view of one embodiment of a compositealumina-carbon agglomerate having a spherical shape and includingdistinct alumina particles embedded in a carbon matrix.

FIG. 3 is a schematic view of one embodiment of a compositealumina-carbon agglomerate.

FIG. 4 is a schematic view of a cylindrical composite alumina-carbonagglomerate with an exterior carbon shell.

FIG. 5 is a schematic view of a cylindrical composite alumina-carbonagglomerate with an exterior carbon shell and a hollow core.

FIG. 6 is a schematic view of one embodiment of a vapor recovery reactorwith a mixed-feed.

DETAILED DESCRIPTION

Reference will now be made to the accompanying figures, which at leastassist in illustrating various pertinent features of the presentinvention. Referring now to FIG. 1, one embodiment of a system forcarbothermically producing aluminum is provided. The system includes amain carbothermic furnace 10 and a vapor recovery reactor 20 (alsoreferred to herein as a VRR). As described in further detail below,during operation of the carbothermic furnace 10, off-gases 30 (e.g., Al(v) and Al₂O (v)) are generated and are provided to the vapor recoveryreactor 20 via one or more conduits (not illustrated). An alumina-carbonfeed 50 is fed to the vapor recovery reactor 20 to react with theoff-gases 30, thereby producing recyclable material 40 (e.g., aluminumcarbide, aluminum carbide slag, alumina). Other materials 70 (e.g.,carbon monoxide gas) may exit the VRR. A separate carbon feed 60 mayoptionally be fed to the vapor recovery reactor 20 to assist in theproduction of the recyclable material 40.

The main carbothermic furnace 10 may be any suitable furnace capable ofcarbothermically producing aluminum. For example, the carbothermicfurnace 10 may be a single zone batch reactor, such as disclosed in U.S.Patent Application Publication No. 2006/0042413 to Fruehan, which isincorporated herein by reference in its entirety, or a dual zonereactor, such as disclosed in U.S. Pat. No. 6,440,193 to Johansen etal., which is incorporated herein by reference in its entirety.

In the first step of a carbothermic process using a batch methodology,alumina (Al₂O₃) and carbon (C) are added to the carbothermic furnace 10to produce slag (Al₂O₃—Al₄C₃+Al₄C₃) and excess solid Al₄C₃. Thecarbothermic furnace 10 operates generally at about 1875° C. to 2050° C.during the slag making step. Off-gases 30 may be produced in the slagmaking step and may contain various aluminum species including Al(vapor), aluminum suboxide (Al₂O, vapor) and carbon monoxide (CO). Theseoff-gases 30 may be provided to the vapor recovery reactor 20.

After the slag producing step, a metal making step is undertaken, wherean Al-C alloy upper phase and a lower slag phase are formed by elevatingthe temperature of the carbothermic furnace 10 to between about 2050° C.to about 2100° C. Off-gases 30 may also be produced during the metalforming step, and may also contain aluminum vapor species. The off-gas30 from the metal forming step may also be provided to the vaporrecovery reactor 20. After the metal making step, a metal tapping stepis completed, where the Al—C alloy is removed from the carbothermicfurnace 10 for further processing to produce the aluminum.

The aluminum vapor species that leave the carbothermic furnace 10 duringthe slag making and metal making steps should be recovered and returnedto the carbothermic furnace 10 in order for the overall process to beeconomically viable. Excess energy can also be recovered from theoff-gases 30, which generally exit the carbothermic furnace 10 attemperatures between 1950° C. and 2100° C. For example, the heat ofcondensation (i.e., the standard enthalpy change of condensation) fromcondensation of the aluminum vapor species on the surface of thealumina-carbon feed 50 materials may be used to react an alumina sourceand a carbon source to form at least a portion of the recyclablematerial 40. The condensed aluminum species may also react with thealumina-carbon 50 feed materials.

To capture the aluminum species, the vapor recovery reactor 20 isgenerally utilized. The vapor recovery reactor 20 may be, for example, avapor recovery reactor as described in U.S. Pat. No. 6,530,970 toLindstad, which is incorporated herein by reference in its entirety. Inone embodiment, the bottom of the VRR column bed will directlyinterconnect with the carbothermic furnace 10. The VRR 20 serves atleast two purposes. First, the VRR 20 captures at least some of the Alvapor species via reaction with the alumina-carbon feed 50. For example,the aluminum vapor species may react with carbon of the alumina-carbonfeed 50 to form the recyclable material 40, such as via the belowreaction:

6Al₂O_((g))+2Al_((g))+16.5C_((s))=>3.5Al₄C_(3(s))+6CO(g)   (9)

Second, the VRR 20 recovers chemical and sensible heat energy from theoff-gas 30. In one approach, aluminum vapor species are captured viacondensation. In a related approach, the heat of condensation is used topromote reaction of the alumina-carbon feed 50 to form the recyclablematerial 40, which may contain one or more of the following species inany combination: Al₃C₄, Al₂O₃—Al₄C₃ slag, Al₂O₃+C mixed solids, Al₄O₄C,Al₂OC, Al₂O₃, and unreacted carbon. Thus, production of recyclablematerial 40 is facilitated, and with little or no outside energy input.

As noted above, the alumina-carbon feed 50 is utilized with the VRR 20to produce the recyclable material 40. In one embodiment, acountercurrent configuration is utilized, wherein the alumina-carbonfeed 50 flows countercurrent to the flow of the off-gases 30 in the VRR20 to facilitate production of the recyclable material 40.

The alumina-carbon feed 50 comprises a composite alumina-carbonmaterial, such as alumina-carbon agglomerate. In this regard, energy canbe recovered from the off-gases 30 by reacting composite agglomeratescomprising alumina (Al₂O₃) and carbon (C) to produce carbide (Al₄C₃) orslag in the VRR 20. In this regard, aluminum vapor species created inthe carbothermic alumina reduction process may be collected in the vaporrecovery reactor 20 and treated with alumina-carbon agglomerates tocreate a recyclable material comprising Al₄C₃, which may be fed to thecarbothermic furnace 10 to assist in, for example, the slag making step.

In one embodiment, the alumina-carbon material is agglomerated oraggregated prior to addition to the VRR 20. In one aspect, a binder isused to hold alumina and carbon together. Agglomeration has severaladvantages over a bed of mixed pure materials. Agglomeration uses finepowders of the individual materials to form larger agglomerates for easeof handling. Agglomerates also add structure to the VRR 20 column bedand allow passage of the off-gases 30 through the VRR 20. The intimatecontacting of the fine powders in the agglomerate can give more rapidreaction kinetics as compared to equivalent size agglomerates ofseparate pure materials. Agglomerates may also have more uniformproperties (e.g., density, thermal conductivity, heat capacity), whicheliminate segregation and differential reaction during passage throughthe VRR 20. Agglomerates can be shaped and sized to optimizedistribution and to restrict or minimize gas pressure drop in the VRR 20and facilitate handling/feeding in the VRR 20. Agglomerate composition(e.g., the ratio of alumina to carbon in the agglomerate) can further beadjusted to facilitate reaction with aluminum vapors and production ofaluminum carbide or other desired species.

As the composite alumina-carbon agglomerate flows through the VRR 20, itabsorbs heat and its temperature gradually increases. When theagglomerate reaches the proper temperature, the alumina melts and reactswith the carbon to produce an Al₄C₃/Al₂O₃ slag, absorbing heat in theprocess. At a somewhat higher temperature, this slag self-reacts toproduce solid Al₄C₃, again absorbing heat. In this fashion, the energyvalue (e.g., heat value) of the off-gases can be captured and put touse, such as, for example, equations 9 and 10, below.

51CO_((g))+6Al₂O_((g))+2Al_((g))→7Al₂O₃ _((s))+15C_((s))+36CO_((g))  (10)

51CO_((g))+6Al₂O_((g))+2Al_((g))→1.67Al₄C_(3(s))+3.67Al₂O_(3(s))+46CO_((g))  (11)

Thus, a VRR utilizing an alumina-carbon agglomerate feed can act as apre-reactor to produce at least some of, and possibly the majority of,the aluminum carbide needed for production processes of the carbothermicfurnace.

The carbon source may be any source of carbon that may react withalumina to form the recyclable material 40 via interaction with thealumina source and/or the off-gases 30, and that is suitable foragglomeration with the alumina source. In one embodiment, charcoal isused as the carbon source. In another embodiment, a petroleum source,such as petroleum coke fines, is utilized as the carbon source. Someother useful carbonaceous sources include wood charcoal, metallurgicalcoke, petroleum coke, cokified carbohydrates, and chemically purifiedcoal. Other carbon sources may be used.

The alumina source may be any suitable source of alumina that is adaptedto form the recyclable material 40 via interaction with the carbonsource and/or the off-gases 30, and that is suitable for agglomerationwith the carbon source. When composite pellets are used, the aluminasource may be fine alumina (Al₂O₃, e.g., smelting grade alumina (SGA) orelectrostatic precipitator (ESP) alumina dust), which can be mixed witha carbon source, for example finely divided petroleum coke, and madeinto a pellet. Conventional SGA is about 50-150 microns in diameter.Conventional SGA has been optimized commercially for use in Hallelectrolytic cells and is an economically suitable source of alumina.

Various binders may be used to form the alumina-carbon agglomerates. Inone aspect, the binder is an organic binder. In one approach, acarbohydrate-based binder may be employed, such as aqueous sugarsolutions, corn starch, corn flour, wheat flour, potato starch, blackcane syrup, dextran, dextrin and the like. In another approach, variouslignosulfonates, such as calcium lignosulfonate, ammonia lignosulfonate,and sodium lignosulfonate, may be employed as binders. In anotherapproach, hydrophobic petroleum-based organic binders, such as coal tarpitch, asphalt emulsion, and petroleum pitch, may be utilized. Inanother approach, the binder is an inorganic binder comprising activatedalumina. In a particular embodiment, the binder consists essentially ofactivated alumina and water. One or more binders may be utilized toforma binder system for the alumina-carbon agglomerate.

In one aspect, the mixture used to create the agglomerates comprises notgreater than about 10% weight binder, such as not greater than about 5%weight binder, or even not greater than about 3% weight binder. Thus,the agglomerate may contain not greater than about 10% by weight binder,such as not greater than about 5% weight binder, or even not greaterthan about 3% binder by weight. In a related aspect, the mixture used tocreate the agglomerates comprises at least about 0.5% weight binder,such as at least about 1% weight binder, or even 1.5% weight binder.Thus, the agglomerates may contain at least about 0.5% weight binder, 1%weight binder, or even 1.5% weight binder. The mixture used to createthe agglomerates, or the agglomerates themselves, may thus containbinder in the range of 0.5% to 10% weight binder, such as 0.5% to 5%weight binder, or even 0.5 to 3% weight binder. In one embodiment, themixture used to create the agglomerates, or the agglomerates themselves,contain binder in the range of 1% to 10% weight binder, 1% to 5% weightbinder, or even 1% to 3% weight binder. In one embodiment, the mixtureused to create the agglomerates, or the agglomerates themselves, containbinder in the range of 1.5% to 10% weight binder, 1.5% to 5% weightbinder, or even 1.5% to 3% weight binder.

The alumina and carbon sources may be co-mingled in various ways tocreate the agglomerates. In one embodiment, the alumina source and acarbon source are mixed uniformly. In another embodiment, theagglomerate includes a core containing at least the alumina source andthis core is further surrounded by a shell comprised mainly of (and insome instances consists essentially of) a carbon source.

The alumina-carbon agglomerates may be any suitable size, which may bedependent on the processing conditions of the carbothermic furnace 10and/or the VRR 20. Smaller pellet sizes may more efficiently absorb heatfrom the off-gases 30. Thus, for substantially spherical agglomeratesutilized in a conventional VRR 20 operated at normal operatingconditions, the agglomerates may have a diameter of from about 3 mm toabout 30 mm, such as from about 5 mm to about 20 mm, or even such asfrom about 10 mm to 15 mm. Thus, the agglomerates may have a diameter ofnot greater than about 30 mm and at least about 3 mm.

In a mixed feed approach (e.g., feeding both an alumina-carbon feed 50and a carbon feed 60 to the VRR 20, as described in further detailbelow), smaller agglomerates may coat the additional carbon feed 60 withslag, which may decrease the efficiency and production rate of therecyclable material 40.

A variety of alumina-carbon agglomerate shapes may be used. For example,the agglomerates may be substantially spherical pellets, an embodimentof which is illustrated in FIG. 2. In the illustrated embodiment, thecomposite pellet 100 comprises a matrix 102 of solid carbon source 104with embedded alumina source 106 (e.g., alumina particles). Thecomposite pellet 100 may comprise any suitable ratio of carbon source toalumina source. In one embodiment, the molar ratio of alumina to carboncan range between about 1:3 to about 1:4.5. In this embodiment, theweight ratio of alumina to carbon is from about 1.88 to about 2.83. Inone embodiment, the weight ratio of alumina to carbon is about 2.2 toabout 2.4.

During movement of the pellets 100 through the VRR 20, various phasechanges and/or volumetric changes may occur during production of therecyclable material 40. For example, the alumina-carbon feed 50 mayreact to form aluminum carbide and/or slag, which may be in the form ofa viscous paste-like substance. Furthermore, aluminum vapors maycondense on the surface of the pellets 100. In turn, there is apotential for clogging of the VRR 20 due to liquid build-up and anoverall volumetric decrease. To restrict flow of the produced recyclablematerial, a shell may be utilized in conjunction with the pellets 100.For example, and with reference to FIG. 3, the composite pellet 100 mayfurther be at least partially surrounded by a shell 120. In theillustrated embodiment, the outer surface of the composite pellet 100 iscompletely surrounded by the shell 120. The shell 120 may comprise anymaterial useful in producing the recyclable material 40 and/orrestricting flow of produced recyclable material, such as a carbonsource. In the latter regard, the carbon source may be any of the carbonsources described above, such as petroleum coke, which may be incalcined, green or needled form. In one embodiment, the shell 120consists essentially of a carbon source and a binder. As describedabove, such a shell 120 may facilitate increased conversion of aluminumvapor species to recyclable material 40 via reaction with the shell 120while promoting reaction of the alumina 106 and carbon 104 in the core100 due to heat gained from the condensation of the aluminum vaporspecies on the surface of the pellets 100.

The shell 120 may be eroded during movement through the vapor recoveryreactor 20 due to reaction with aluminum vapor species and/or due tophysical interaction with other pellets 100. As noted above, the size ofthe composite pellet 100 may be tailored to the process parameters ofthe carbothermic furnace 10 and/or the VRR 20. In turn, the shell 120may also be sized in view of these processing parameters and theconsiderations provided above. The thickness of the shell 120 may betailored to facilitate structural support and/or stoichiometry (e.g.,reaction with aluminum vapor species), wherein pellets 100 exiting thevapor recovery reactor 20 include the at least a portion of the shell120. Competing considerations generally dictate shell thickness. Theshell 120 should be thick enough to maintain its structural integrity soas to restrict flow of produced recyclable material (e.g., slag createdtherein). However, providing excess carbon to the carbothermic furnace10 is undesirable during metal making operations, and therefore it isappropriate, in some instances, to restrict the thickness of the shell120. Thus, in some embodiments, the shell 120 has a thickness that isjust thick enough to restrict flow of produced recyclable material, butis thin enough such that little excess carbon is provided to thecarbothermic furnace 10. Hence, the shell 120 may have a first thicknessas it enters the VRR and a second thickness as it exits that VRR, andthis first thickness may be tailored so that the second thickness issubstantially achieved as the pellets exit the VRR.

In another embodiment, the agglomerates may be substantiallycylindrical. One embodiment of a cylindrical agglomerate is illustratedin FIG. 4. In the illustrated embodiment, the cylindrical agglomerate200 comprises a matrix 202 of solid carbon source 204 with embeddedalumina particles 206, such as described above with respect to FIG. 2.In the illustrated embodiment, the cylindrical agglomerate 200 alsoincludes a shell 220, such as described above with respect to FIG. 3.However, the cylindrical agglomerate 200 may be utilized without theshell 220.

Cylindrical agglomerates 200 comprising a shell 220 may be useful, forinstance, when structural support is desired, which may be provided bythe shell 220, or when it is desired to segregate the alumina-carbonmatrix from another carbon feed 60 (e.g., charcoal). In the latterregard, as the agglomerates 200 move through the VRR 20, thealumina-carbon matrix may react to form a viscous recyclable material,which could flow and contact the another carbon feed 60, resulting inrestricted interaction of aluminum vapor species with the another carbonfeed 60. To restrict the viscous recyclable material from contacting theanother carbon feed 60, the shell 220 may be used to contain theproduced viscous recyclable material. Furthermore, the shell 220 mayrestrict or prevent clogging of the VRR as the agglomerates 200 formrecyclable material 40. When the cylindrical agglomerate 200 isconverted to recyclable material 40, such as Al₄C₃, its volume maydecrease. Due to the decreased volume, the bulk porosity of the bedwithin the VRR 20 may increase. Furthermore, if the recyclable material40 is sufficiently viscous, it may cause clogging of the VRR 20. Theshell 220 may prevent produced recyclable material from flowing outsideof the shell 220, thereby restricting, and in some instances preventing,clogging of the VRR 20. Additionally, the shell 200 may assist inmaintaining the bulk porosity of the VRR by restricting flow of theproduced recyclable material. In one embodiment, additional carbon maybe used in the agglomerate core and/or shell so as to facilitatemaintenance of the structural integrity of the agglomerates.

The thickness of the shell 220 may be tailored depending upon thedesired application. For example, the thickness may be sufficientlythick so as to facilitate structural integrity of the shell 220 as theagglomerate 200 flows through the VRR. The shell thickness may also besufficiently thin so as to restrict the amount of unreacted carbon shellmaterial, or other shell material, entering the carbothermic furnace 10after exiting the VRR 20.

In a related embodiment, the cylindrical agglomerates 200 may comprise ahollow core, such as illustrated in FIG. 5. The hollow core 240 mayfacilitate heat and mass transfer to the interior of the pellet 200,thereby promoting production of the recyclable material 40.

In addition to sizing and shaping the agglomerates to as to facilitateheat transfer, mass transfer, kinetics and thermodynamics, theagglomerates should also be sufficiently strong so as to withstand theweight of the column in the VRR 20. Crush strength represents theresistance of a solid to compression forces. For a conventional VRR 20,the agglomerates may have a single agglomerate crush strength suitableto withstand normal crush forces experienced in at least some of thevapor recovery reactor 20.

The agglomerates may be manufactured by any suitable process. Forexample, a milling process may be used, where an alumina source and acarbon source are mixed with an organic binder. Water may also beoptionally added, depending on the binder selected. The raw mixture maythen be pelletized into green pellets, such as by wet tumbling with aballing disc or by a balling drum pelletizer. Subsequently, the greenpellets may be hardened and/or dried. The temperature may be regulatedby, for example, use of exhaust gas, heat-exchanged air or nitrogen gas.An extrusion process may also be utilized, such as when producingagglomerates comprising an outer carbon shell. Other methods ofproducing agglomerates may also be employed, such as via press andslurry techniques,

As noted above, the alumina-carbon feed 50 may be utilized to facilitateproduction of the recyclable material 40 from the off-gases 30. Inanother embodiment, the alumina-carbon feed 50 may be utilized inconjunction with the carbon feed 60 to facilitate production of therecyclable material. More particularly, a mixed feed comprising both acarbon feed 60 and alumina-carbon feed 50 may be utilized. In operation,the energy value of the off-gas 30 is used to drive carbide formation inthe alumina-carbon feed 50. Concomitantly, the separate carbon feed 60reacts with aluminum vapor species to produce aluminum carbide.

One embodiment of a mixed feed configuration is illustrated in FIG. 6.In the illustrated embodiment, the vapor recovery reactor 20 receivesoff-gases 30 from the carbothermic furnace 10 (not illustrated). Thevapor recovery reactor 20 also receives a mixed feed comprising analumina-carbon feed 50 and a carbon feed 60. The alumina-carbon feed 50may be any of the above-described alumina-carbon agglomerates, includingthe illustrated alumina-carbon pellets 52. The carbon feed 60 may be anysuitable carbonaceous material. In the illustrated embodiment, thecarbon source includes a first carbonaceous material 62 and a secondcarbonaceous material 64. The first carbonaceous material 62 may be, forexample, a carbon-containing agglomerate having a first porosity. Thesecond carbonaceous material 64 may have a higher porosity relative tothe first carbon material 62. In the illustrated embodiment the firstcarbon source 62 is a carbon pellet and the second carbon source 64 is acarbon ring. As noted above, the VRR 20 may become clogged if excessslag and/or condensation occurs. To restrict clogging and facilitate gasflow through the VRR, carbonaceous rings 64 may be utilized within theVRR. The addition of carbon rings 64 will give the VRR greater porosityand more openings for gas flow. Carbon rings 64 will travel through themoving bed partially reacting with aluminum vapor species, but willmaintain structural integrity. Carbon rings can be mass-produced byextruding a pet coke/pitch mixture through circular dies and thencutting the extruded cylinders off with a revolving blade. The carbonrings are then calcined, such as in situ in cooler portions of the VRR.The optimum size and shape of the rings may be optimized to provide highstrength and porosity for the least amount of carbon. Thinner-wall ringsmay be more desirable if structural integrity is maintained. Longerrings might pack more like straw, providing greater VRR bed porosity andleaving an open structure if the alumina-carbon pellets 52 and/or othercarbon pellets 62 collapse. Thus, the carbon feed 60 may include aplurality of different carbon sources to facilitate production ofrecyclable material 40 and mass transfer of gases through the vaporrecovery reactor 20.

As noted above, the carbon feed 60 may include one or more of theabove-described carbonaceous materials. One useful carbonaceous materialis charcoal. Charcoal has a high surface area and a high reactivity withthe Al(g) and Al2O, and thus readily produces slag or solid Al4C3.Charcoal also has a high crush strength, which may provide structuralintegrity to the bed. When utilizing charcoal as at least a portion ofthe carbon feed 60, particle size may be considered. Optimal charcoalparticle diameter varies from application to application. A smallerparticle diameter leads to a greater conversion of carbon to carbide,but also leads to a greater pressure drop within the VRR and possibly anincrease in the likelihood of clogging the column. Furthermore,stoichiometry may be evaluated. Complete conversion of charcoal may notbe desirable since charcoal may provide structural integrity to the VRRbed. In one aspect, the carbon feed 60 may consist essentially ofcharcoal briquettes. In another aspect, another carbonaceous material(e.g., pet coke and/or pitch) could be utilized within one or morecharcoal briquettes.

In one embodiment, charcoal may be generated in situ within the VRR 20.For example, the carbon feed 60 may comprise saw dust, crumbled bark,pitch or other cellulosic material, wherein the energy value of the offgases 30 is utilized to dehydrate and react such cellulosic materialinto charcoal. In situ charcoal generation is described in Fruehan etal., Metall.Mater. Trans. B, 35B, pp 617-623, 2004, which is hereinincorporated by reference. Furthermore, the shape and size of the insitu generated charcoal could be tailored. Knowing the charcoalizationshrinkage, it would be possible to tailor the characteristics of thefinal charcoal briquette. In one aspect, the in situ charcoal could begenerated in a specific size or shape. For example, the size and shapeof a wood agglomerate could be extruded into rings to produce carbonrings 64, described above. Furthermore, the porosity of the producedcharcoal could be controlled on various levels, from the micro-porosityof the wood cell lumen to the macro-porosity of the agglomeratedsaw-dust particles, to achieve the desired porosity within the VRRduring operation of the reactor.

As noted above, the alumina-carbon feed is generally utilized with theVRR 20 to facilitate recovery of aluminum vapor species. However, it isanticipated that at least some of the alumina-carbon feed could also befed directly to the carbothermic reactor 10 (e.g., during slag making)to promote capture of aluminum vapor species within the reactor.

Having described certain embodiments, it is to be understood that theinvention may be otherwise embodied within the scope of the appendedclaims. Whereas particular embodiments of this invention have beendescribed above for purposes of illustration, it will be evident tothose skilled in the art that numerous variations of the details of thepresent invention may be made without departing from the invention asdefined in the appended claims.

1. A method for recovering off-gases from a carbothermic aluminumfurnace, the method comprising: passing, in a vapor recovery reactor, afeedstock countercurrent to off-gases from a carbothermic aluminumfurnace, the feedstock comprising an alumina-carbon agglomerate;treating, concomitant to the passing step, the off-gases with thefeedstock; and recovering at least some recyclable material for re-usein the carbothermic aluminum furnace.
 2. The method of claim 1, whereinthe treating step further comprises heating the feedstock with theoff-gases.
 3. The method of claim 1, wherein the feedstock furthercomprises a carbon feed.
 4. The method of claim 3, wherein the carbonfeed comprises a carbon agglomerate.
 5. The method of claim 4, whereinthe carbon agglomerate is one of a sphere, a briquette, a ring, and acylinder.
 6. The method of claim 5, wherein the carbon agglomerateconsists essentially of wood charcoal, metallurgical coke, petroleumcoke, cokified carbohydrates, chemically purified coal, and mixturesthereof.
 7. The method of claim 4, further comprising: generating thecarbon agglomerate in situ in the vapor recovery reactor.
 8. The methodof claim 1, wherein the passing step comprises: feeding the feedstockfrom a hopper; and pushing the feedstock toward an inlet of the vaporrecovery reactor via a screw-drive.
 9. The method of claim 1, whereinthe treating step comprises: reacting the off-gases with a carbonmaterial of the alumina-carbon agglomerate, thereby forming therecyclable material.
 10. The method of claim 1, wherein the treatingstep comprises: condensing the off-gases on a surface of thealumina-carbon agglomerate.
 11. The method of claim 1, wherein thealumina-carbon agglomerate comprises a core and a shell substantiallysurrounding the core, wherein the core comprises alumina and the shellcomprises carbon, wherein the treating step comprises: reacting theoff-gases with the carbon of the shell; exposing the core of thealumina-carbon agglomerate; and reacting the alumina of the core withcarbon.
 12. The method of claim 1, wherein the recyclable materialcomprises at least one of aluminum carbide and slag.